Current interrupting brush holder assembly

ABSTRACT

A method and apparatus are disclosed for preventing damage to an electric motor due to worn commutator brushes. The invention interrupts the current flow from a power source to the motor when the brushes wear down to a predetermined size. According to a first embodiment, a trip member is released from a brush arm to open the electrical circuit. A second embodiment causes the worn brushes to be discharged from the brush holder assembly, while a third embodiment employs a wedge contact which moves away from the worn brush to open the electrical circuit.

United States Patent Vassos et al. Aug. 5, 1975 [54] NT INTERRUPTING BRUSH 3,480,814 11/1969 Amrein 310/247 HOLDER ASSEMBLY 3,513,343 5/1970 Harvey 310/247 3,523,288 8/1970 Thompson..,. 310/239 1 5] Inventors: Louis J- Vassos, k R g 3,534,200 10/1970 Carey 310 249 William P. Elzer, Harwood Heights; 3,710,160 1/1973 Voglesonger 310/247 Alex F. Gawron, Chicago, all of 111.

[73] Assignee: Skil Corporation, Chicago, 111.

[22] Filed: Jan. 21, 1974 [21] App]. N0.: 435,323

[52] US. Cl. 310/242 [51] Int. Cl. H02k 13/00 [58] Field of Search 310/239, 240, 242, 245, 310/241, 246, 247, 249

[56] References Cited UNITED STATES PATENTS 2,775,717 12/1956 Laing 310/246 2,842,693 7/1958 Tubbs... 310/240 2,851,622 9/1958 Hall 310/246 3,108,201 10/1963 Summerfield.... 310/247 3,466,481 9/1969 Sckerl 310/239 Primary Examiner-R. Skudy Attorney, Agent, or Firn1McDougall, Hersh & Scott [57] ABSTRACT A method and apparatus are disclosed for preventing damage to an electric motor due to worn commutator brushes. The invention interrupts the current flow from a power source to the motor when the brushes wear down to a predetermined size. According to a first embodiment, a trip member is released from a brush arm to open the electrical circuit. A second embodiment causes the worn brushes to be discharged from the brush holder assembly, while a third embodiment employs a wedge contact which moves away from the worn brush to open the electrical circuit.

14 Claims, 19 Drawing Figures PATENTEBAUG 5l975 3. 898.492

SHEET 2 CURRENT INTERRUPTING BRUSH HOLDER ASSEMBLY BACKGROUND OF THE INVENTION The present invention relates to commutator brush assemblies for electrical motors. In a standard brush assembly, such as disclosed in the US. Pat. to Summerfield No. 3,108,201, when the brushes wear down to a selected size they are normally restrained by the brush holder assembly. This is to prevent the brushes from wearing completely away and allowing the spring and field wire to come in contact with the commutator. Should the spring or field wire touch the commutator, they can pit and gash it, requiring replacement of the motor.

Restraining the brush in this manner, however, interferes with good electrical contact between the brush and the commutator. If the brush does not firmly contact the commutator, arcing results. After a short period of time this arcing will cause a temperature rise and may result in the distortion of the brush holder. If arcing continues, the commutator becomes heavily pitted and the motor sustains irreparable damage. It is therefore desirable to provide a brush holder assembly capable of automatically turning an electrical motor off prior to its brushes wearing down to the destructive stage.

It is accordingly an object of the present invention to provide a method and apparatus for automatically turning off an electric motor when the brushes have worn down to a predetermined size.

It is a further object of the present invention to provide a brush holder assembly of simplified construction facilitating rapid assembly.

It is a still further object of the present invention to provide a brush holder assembly which will prevent damage to an elecctric motor commutator.

Other objects of the invention will become apparent from the concluding portion of the specification.

SUMMARY OF THE INVENTION According to the present invention, a brush holder assembly includes brush means positioned within the holder for conducting current from a power source to the commutator of a motor and means for disconnecting the brush from the power source when it wears down to a predetermined size. In a first embodiment, a trip member, conducting current from the power source, is retained by a brush arm in electrical contact with the brush means. When the brush means wears down to the selected size, the trip member escapes from the brush arm thereby interrupting the current flow.

In a second embodiment, the brush means is positioned within the brush holder and urged against the commutator by a spring. A contact member positioned between the spring and the brush means conducts current from the source to the brush. When the brush wears down to the predetermined size it is released from the holder assembly by the rotation of the commutator, the contact member being restrained by stops located in the holder.

In a third embodiment, a brush means is urged against the commutator by a spring. Positioned between the spring and the brush means is a wedge contact conducting current from the power source. When the brush means wears down to the predetermined size the wedge contact is urged by the spring into an auxiliary channel in the brush holder and out of electrical contact with the brush means.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal section through a brush holder according to a first embodiment.

FIG. 2 is an end view of the brush holder of FIG. 1 having portions broken away.

FIG. 3 is a longitudinal cross-section similar to FIG. 1 showing the brush arm in operative position. FIG. 4 is a cross-sectional view taken along the lines 44 of FIG. 3.

FIG. 5 is a longitudinal cross-section similar to FIG. 1 showing the trip member disengaged from the brush arm.

FIG. 6 is a perspective view of the trip member according to the first embodiment.

FIG. 7 is a longitudinal cross-section of a brush holder assembly according to a second embodiment showing the brush means in its initial position.

FIG. 8 is an end view of the brush holder of FIG. 7.

FIG. 9 is a longitudinal cross-section similar to FIG. 7 showing the brush means and Contact member just prior to interrupting current flow.

FIG. 10 is a cross-sectional view taken along the lines 10-10 in FIG. 9.

FIG. 11 is a partial cross-sectional view similar to FIG. 7 showing the brush being moved out of the holder assembly by the rotation of the commutator.

FIG. 12 is a perspective view of the contact member employed in the second embodiment.

FIG. 13 is a longitudinal cross-section of a brush holder assembly according to a third embodiment.

FIG. I4 is an end view of the brush holder of FIG. 13.

FIG. 15 is a cross-sectional view similar to FIG. 13 showing the wedge contact beginning to move out of contact with the brush.

FIG. 16 is a cross-sectional view taken along the lines 16-16 in FIG. 15.

FIG. I7 is a partial cross-sectional view similar to FIG. 13 showing the wedge contact just prior to interrupting current flow to the brush. FIG. 18 is a crosssectional view similar to FIG. 13 showing the wedge contact completely out of the main channel of the brush holder.

FIG. 19 is a perspective view of the wedge contact according to the third embodiment.

DETAILED DESCRIPTION Referring to FIGS. 1-6, a brush assembly 20 according to a first embodiment is shown. A brush holder 22 preferably formed of a plastic material has a main channel 24 located therein. A neck portion 26 separates the main channel 24 from an upper chamber 28. Positioned within the main chamber 24 is a commutator brush 30 adapted to contact a commutator 32 of an electric motor. A current conducting brush arm 34 is press fitted to one end of brush 30. A coil spring 36 urges the brush and arm into electrical contact with the commutator 32.

Mounted in the upper chamber 28 of brush holder 22 is an L-shaped trip member 38. End 40 of the trip member extends externally of the brush holder for connection to a power source not shown. The other end 41 is disposed within the upper chamber at an acute angle to the horizontal. As best seen in N04, the trip member 38 is designed to be received in an aperture 42 in the brush arm 34. When the trip member is so engaged, a current path exists from the trip member through the brush arm 34 to brush 30 and to the commutator 32.

When the brush assembly is being installed, it is desirable to temporarily locate the brush arm 34 in the extreme rearward position shown in FIG. 1. For this purpose a locking tab 44 is provided on the trip member. This position facilitates assembly of the unit by restraining the coil spring while the brush and armature are installed. On completing assembly, the trip member 38 is depressed sufficiently to release the brush arm 34 from the tab. The unit is then ready for operation and appears as illustrated in FIG. 3.

During operation of the motor, the brush, being preferably formed of carbon, will wear against the armature. This reduction in size is compensated for by the expansion of coil spring 36 thereby maintaining good electrical contact. The brush and brush holder thus move horizontally in the main channel 24 towards the commutator 32 due to the action of the coil spring.

When the brush 30 wears down to a predetermined size, as shown in FIG. 5, the brush arm will reach a point just beyond 41 of the trip member 38 releasing the trip member from the aperture 42 of the brush arm. The trip member is mounted in the upper chamber at a slight angle to the horizontal sufficient to bias it away from the brush arm. Accordingly, when the brush arm moves beyond end 41, the trip member rapidly moves away from and out of contact with the brush arm, as indicated by the arrow in FIG. 5. This is effective for interrupting current flow to the motor.

Thus, when a brush reaches a predetermined size the brush holder causes the tool to turn off and remain off until a new brush is installed and the trip member is once again captured in the aperture 42. This prevents damage to the motor which would ordinarily be caused by continuing to use the tool when a brush is completely worn out or is not being maintained in good electrical contact with the commutator.

Referring now to FIGS. 2-12, a second embodiment of the present invention is disclosed. As with the first embodiment, when the brush 30 wears down to a predetermined size, the current path to the commutator 32 is interrupted for preventing further operation until new brushes are installed. Referring specifically to FIG. 7, a brush housing 50 has a channel 52 adapted to receive a coil spring 54, a current conducting contact member 56 and a brush 30. In this embodiment the current conducting contact member 56 is not bonded to the brush 30 but is merely sandwiched between the spring and the brush. A notched portion 58 and a tab portion 60 are adapted to engage stops 62 and 64 provided on the housing 50 as will be explained. Current is supplied to the brush 30 from contact member 56 which is connected to a cable 61 from the power source.

In operation, the brush and contact member 56 move toward the commutator as the brush wears down to maintain good electrical contact therewith. When the brush 30 has worn down to the size shown in FIGS. 9 and 10, the brush will be outside of the brush holder and unsupported. As shown in FIG. 11, the brush will therefore rapidly rotate away from the brush holder due to its frictional engagement with the commutator. As soon as the brush is gone, the notch 58 and tab 60 of the contact member 56 will engage the stops 62 and 64 preventing further movement of the contact member toward the commutator. An insulating air gap will therefore exist between the commutator and the contact member 56, for interrupting the current flow. To facilitate the removal of the brush from the holder when it has worn down, the mating surface 66 of contact member 56 and the brush end may be curved.

The brush shown in FIG. 11 will be flung away in one piece. Alternately, if desired, the back portion of the brush may have slots cut into it at an appropriate depth. In this design, when the brush reaches the stage shown in FIG. 11, the commutator will have worn the brush down to the slots and it will break up into pieces as it moves out of the brush holder.

Referring now to FIG. 13-19, a brush holder assembly according to a third embodiment is shown. A housing 70 has a main channel 72 containing a coil spring 74, a wedge contact 76 and a brush 78. A compartment 80 adapted to receive the wedge contact 76 therein is located at one end of the housing 70. Current is supplied from a power source via cable 82 through the wedge contact 76 to the brush 78 and commutator 32.

The wedge contact 76 is beveled along a bottom surface thereof and has an insulating layer 84 affixed to the beveled surface. As shown in FIG. 13, the spring 74 urges the brush 78 into contact with the commutator and at the same time urges the wedge contact 76 upwardly. Prior to the brush wearing down to a predetermined size the wedge is prevented from moving upwardly due to a pair of ledges 86 and 88 abutting against shoulder 90 and 92 of the wedge contact (FIG. 19). These ledges extend parallel to the main chamber.

Initially the spring 74 is substantially compressed and the wedge contact is in electrical contact with the brush 78. As the brush wears down the spring expands and the wedge contact and brush move towards the commutator as shown in FIG. 15. As shown in FIG. 17, when the brush is worn down to the predetermined size the wedge contact 76 is released from the ledges 86 and 88 and begins moving upwardly into the chamber 80. The electrical contact surface area between the brush and wedge begins to decrease along the interface 81. In FIG. 18 the wedge contact has moved rapidly and completely out of the main channel into the chamber 80 thereby to interrupt current flow to the commutator. The insulating layer 84 provided along the beveled surface of the wedge contact prevents a sneak current path through the coil spring 74 to the brush and armature. The movement of the wedge out of contact with the brush requires at the most approximately one second to clear the ledges and pop out of the main channel.

While we have shown and described an embodiment of this invention in some detail, it will be understood that this description and illustration are offered merely by way of example, and that the invention is to be limited in scope only by the appended claims.

We claim:

1. A brush holder assembly which interrupts current flow from a power source to an electric motor commutator comprising:

brush means positioned within said holder for conducting current from said source to said commutator; and

means for automatically and affirmatively disconnecting said brush means from said power source in response to the former wearing down to a predetermined size.

2. The brush holder of claim 1 wherein:

said brush means includes a brush, and a current conducting brush arm attached to said brush;

said disconnecting means includes a current conducting trip member retained by said brush arm for providing said current flow and which is released from said brush arm when said brush wears down to said predetermined size.

3. The brush holder of claim 2 wherein said trip member is resiliently biased away from said brush arm; said brush arm having an aperture for retaining said trip member therein prior to said brush wearing down to said predetermined size.

4. The brush holder of claim 2 wherein said trip member has a locking tab thereon for temporarily securing said brush arm during assembly of said brush holder, said brush arm being released from said tab prior to operating said motor.

5. The brush holder of claim 1 wherein said brush means includes:

a current conducting brush contacting said commutator and movable in said holder;

said brush moving out of said holder to thereby discontinue said current flow when said brush wears down to said predetermined size.

6. The brush holder of claim 1 wherein said holder further includes means for urging said brush means toward said commutator.

7. The brush holder of claim 5 wherein said brush is moved out of said holder by the rotation of said commutator.

8. The brush holder of claim 5 further including:

a contact member for conducting current from said source to said brush, said member moving with said brush;

stops positioned within said holder for restraining said contact member from contacting said commutator when said brush moves out of said holder.

9. The brush holder of claim 1 wherein said means for disconnecting includes:

a wedge contact for conducting current from said source to said brush means until said predetermined size is reached, said wedge contact thereafter moving in a direction away from said brush means and out of electrical contact therewith to discontinue current flow.

10. The brush holder of claim 9 wherein said brush means includes:

a brush; and means for urging said brush toward said commutator;

said urging means also causing said wedge contact to move away from and out of electrical contact with said brush when said predetermined size isreached.

11. The brush holder of claim 10 wherein a surface of said wedge contact is electrically insulated to prevent undesired current flow through said urging means to said brush when said wedge contact is out of electrical contact with said brush.

12. A brush holder assembly which automatically interrupts current flow from a power source to an electric motor commutator comprising:

a. a brush positioned within said holder for contacting said commutator;

b. a wedge contact positioned in said holder adatped to conduct current from said source to said brush;

c. spring means for urging said brush into electrical contact with said commutator and for causing said wedge contact to move away from and out of electrical contact with said brush when the latter has worn down to a predetermined size.

13. The brush holder of claim 12 wherein said brush holder includes:

a main channel containing said brush, wedge contact and spring means; and

an auxiliary channel communicating with said main channel, said spring means urging said wedge contact out of said main channel into said auxiliary channel to thereby interrupt said current flow.

14. The brush holder of claim 13 wherein a pair of ledges disposed in said main channel restrain said wedge contact therein until said brush has reached said predetermined size. 

1. A brush holder assembly which interrupts current flow from a power source to an electric motor commutator comprising: brush means positioned within said holder for conducting current from said source to said commutator; and means for automatically and affirmatively disconnecting said brush means from said power source in response to the former wearing down to a predetermined size.
 2. The brush holder of claim 1 wherein: said brush means includes a brush, and a current conducting brush arm attached to said brush; said disconnecting means includes a current conducting trip member retained by said brush arm for providing said current flow and which is released from said brush arm when said brush wears down to said predetermined size.
 3. The brush holder of claim 2 wherein said trip member is resiliently biased away from said brush arm; said brush arm having an aperture for retaining said trip member therein prior to said brush wearing down to said predetermined size.
 4. The brush holder of claim 2 wherein said trip member has a locking tab thereon for temporarily securing said brush arm during assembly of said brush holder, said brush arm being released from said tab prior to operating said motor.
 5. The brush holder of claim 1 wherein said brush means includes: a current conducting brush contacting said commutator and movable in said holder; said brush moving out of said holder to thereby discontinue said current flow when said brush wears down to said predetermined size.
 6. The brush holder of claim 1 wherein said holder further includes means for urging said brush means toward said commutator.
 7. The brush holder of claim 5 wherein said brush is moved out of said holder by the rotation of said commutator.
 8. The brush holder of claim 5 further including: a contact member for conducting current from said source to said brush, said member moving with said brush; stops positioned within saId holder for restraining said contact member from contacting said commutator when said brush moves out of said holder.
 9. The brush holder of claim 1 wherein said means for disconnecting includes: a wedge contact for conducting current from said source to said brush means until said predetermined size is reached, said wedge contact thereafter moving in a direction away from said brush means and out of electrical contact therewith to discontinue current flow.
 10. The brush holder of claim 9 wherein said brush means includes: a brush; and means for urging said brush toward said commutator; said urging means also causing said wedge contact to move away from and out of electrical contact with said brush when said predetermined size is reached.
 11. The brush holder of claim 10 wherein a surface of said wedge contact is electrically insulated to prevent undesired current flow through said urging means to said brush when said wedge contact is out of electrical contact with said brush.
 12. A brush holder assembly which automatically interrupts current flow from a power source to an electric motor commutator comprising: a. a brush positioned within said holder for contacting said commutator; b. a wedge contact positioned in said holder adatped to conduct current from said source to said brush; c. spring means for urging said brush into electrical contact with said commutator and for causing said wedge contact to move away from and out of electrical contact with said brush when the latter has worn down to a predetermined size.
 13. The brush holder of claim 12 wherein said brush holder includes: a main channel containing said brush, wedge contact and spring means; and an auxiliary channel communicating with said main channel, said spring means urging said wedge contact out of said main channel into said auxiliary channel to thereby interrupt said current flow.
 14. The brush holder of claim 13 wherein a pair of ledges disposed in said main channel restrain said wedge contact therein until said brush has reached said predetermined size. 